The present invention relates to ion conducting membranes (ICMs) and more specifically, ICMs that are used in Polymer Electrolyte Membrane (PEM) fuel cells.
In PEM fuel cell applications, a proton conductive membrane is placed between two electrodes, an anode and a cathode, and in some cases the electrodes are bonded directly to the membrane. Protons are conducted through the ICM from the anode to the cathode, and the conductance of the membrane affects the performance and power density of the fuel cell. In order to improve the performance of fuel cells, the resistance of the ICM must be reduced. One means to reduce the resistance is to reduce the thickness of the ICM. However with extruded or cast films of ionomer, the strength of the layer is reduced along with the thickness, making the films less dimensionally stable and difficult to handle.
A reinforced ion exchange membrane is described in JP11067246 to Asahi Glass. In this invention, the ICM is reinforced with fluorocarbon polymer fibers woven into a cloth consisting of warp and weft fibers between 10 and 100 denier. This composite provides increased strength for thin membrane composites.
A composite ICM described in U.S. Pat. No. 5,547,551 to Bahar, et. al. describes a base material and an ion exchange resin. The base material is a membrane which is defined by a thickness of less than 1 mil (0.025 mm) and a microstructure characterized by nodes interconnected by fibrils, or a microstructure characterized by fibrils with no nodes present. The ion exchange resin substantially impregnates the membrane such that the membrane is essentially air impermeable. In an integral composite membrane the strength is enhanced by the microporous membrane allowing further reduction of the thickness of the layer, and thereby lowering the resistance to proton transport. These thin integral composite membranes thus are able to provide lower resistance, while maintaining high strength.
The PEM fuel cell system is however a very aggressive environment for any thin membrane. In order to minimize electrical contact resistance between the components, the cells are typically compressed to approximately 50 to 400 psi. At these high pressures, thin membranes are susceptible to electrical shorting across the electrodes. In addition, at high pressures longer term mechanical stability properties become important for the ICM. Although the microporous reinforcement of the ICM enhances strength, which reduces swelling and tearing, known reinforcements do not provide adequate through-plane resistance to puncture. As used herein, xe2x80x9cin-planexe2x80x9d means parallel to the plane of the sheet of material, and xe2x80x9cthrough-planexe2x80x9d means vertically through the plane of the sheet of material.
On either side of the ICM there is typically an electrode comprised of carbon particles, ionomer, and catalyst. Outside of the electrode layers a gas diffuison layer comprising carbon fibers, in either a woven or non-woven form, is usually placed. In some cases, a micro layer is applied to the gas diffusion media which comprises carbon or graphite particles, ionomer and fluoropolymer. Most gas diffusion media is very rough relative to the ICM layer. In addition, the thickness variability of the gas diffusion media can be large, especially for woven type gas diffusion media. Thickness variations of 0.002 to 0.004xe2x80x3 are common for woven type gas diffusion media. Non-uniformity of gas diffusion media thickness can lead to high pressure areas upon assembly. These high pressure areas can cause thinning of the ICM and in some cases electrical shorting across the anode and cathode. Furthermore, the rigid carbon fibers of the gas diffusion media can puncture through the ICM and create an electrical short during fuel cell assembly or some time later as a result of continued penetration of the fiber through the ICM over time. Fuel cells are normally run at an elevated temperature that can accelerate compressive mechanical creep of ionomers. Such creep may also thin the ICM, increasing the propensity of the fibers from the gas diffusion media to penetrate through the ICM layer.
Another challenge for thin ICM is compressive creep around electrode thickness non-uniformities. In cases where the electrode layers are not uniform in thickness, the pressure exerted on the ICM can vary dramatically. In the thick electrode areas where the pressure is elevated, creep may occur more rapidly, further thinning the ICM layer. Eventually, the compressive creep of the ICM may result in anode to cathode contact and an electronic short across the membrane.
Electronic shorts through the ICM result in reduced efficiency of the fuel cell. The voltage drop across the electronic short through the membrane has to be the same as the operating voltage of the cell. Therefore, a corresponding current is drawn away from the fuel cell and is carried through the electronic short. The lower the resistance value of the short, the higher the associated current.
It is therefore necessary to provide a thin ICM that retains the low ionic resistance but that more effectively resists puncture and subsequent shorting. Another necessity is to minimize the in-plane dimensional changes as a function of hydration. Through-plane hydration expansion is a desired property however, as it will further reduce contact resistance between components in the fuel cell.
The present invention is a distinct improvement over the previously known ion conducting composite membranes, in that it has increased hardness and dimensional stability. In one embodiment of the present invention, an integral composite membrane comprised of an expanded polytetrafluoroethylene having a morphological structure comprising a microstructure of very highly elongated nodes interconnected by fibrils is imbibed with ionomer. This composite membrane shows a surprising increase in hardness and thereby reduces electrical shorting and improves fuel cell performance and durability.
Specifically, the present invention provides a composite membrane formed of (a) an expanded polytetrafluoroethylene membrane having an internal microstructure of nodes interconnected by fibrils, the nodes aligned substantially in parallel, being highly elongated and having an aspect ratio of 25:1 or greater; and (b) an ion exchange material impregnated throughout the membrane, the impregnated expanded polytetrafluoroethylene membrane having a Gurley number of greater than 10,000 seconds, wherein the ion exchange material substantially impregnates the membrane so as to render an interior volume of the membrane substantially occlusive.
In another aspect, the present invention provides a composite membrane comprising a base material having a microstructure of nodes and fibrils forming interconnected passages and pathways and having a hardness greater than 1000 mPa, and an ion exchange material impregnated throughout the base material, the composite membrane having a Gurley number of greater than 10,000 seconds, wherein the ion exchange material substantially impregnates the base membrane so as to render the passages and pathways substantially occlusive.